Previously, it has been known that a polystyrene-based resin expanded molded article, obtained by filling polystyrene-based resin pre-expanded beads in a mold, and heating and expanding, is excellent in stiffness, thermal insulating property, lightweight property, water resistance and expansion moldability. For this reason, this expanded molded article is widely used as a cushioning material or a thermal insulating material for a building material. However, this expanded molded article has a problem that it is inferior in chemical resistance and impact resistance.
On the other hand, it is known that an expanded molded article comprising a polyolefin-based resin such as a polyethylene-based resin and a polypropylene-based resin is excellent in chemical resistance and impact resistance. For this reason, this expanded molded article is used in automobile-related parts. However, since the polyolefin-based resin is inferior in retainability of a blowing gas, and it is necessary to precisely control expansion molding conditions, there is a problem that the manufacturing cost is high. In addition, there is also a problem that stiffness is inferior as compared with the polystyrene-based resin expanded molded article.
In order to overcome the problems of the polystyrene-based resin and the polyolefin-based resin, there is reported an expanded molded article obtained by combining the polystyrene-based resin having better stiffness and expansion moldability, and the polyolefin-based resin having better chemical resistance and impact resistance.
Nature of this combined expanded molded article is greatly influenced by a ratio of the polystyrene-based resin and the polyolefin-based resin. That is, as a ratio of the polyolefin-based resin is higher, chemical resistance and impact resistance of an expanded molded article are improved, but stiffness and expansion moldability are lowered.
In particular, when the expanded molded article is used in automobile-related parts, since there is a possibility that the article is contacted with chemicals such as gasoline, kerosene, brake oil and vinyl chloride plasticizer, and undergoes strong impact, high chemical resistance and impact resistance are required. As a method satisfying this requirement, there is known a method of improving chemical resistance and impact resistance of the expanded molded article by adjusting a polyolefin-based resin component in an expanded molded article at 50% by weight or more. However, in this method, since an amount of the polystyrene-based resin component is relatively reduced, stiffness and expansion moldability of the expanded molded article are greatly lowered. As a result, this expanded molded article could not be widely used in automobile-related parts.
Then, for the purpose of mutually compensating drawbacks of the polystyrene-based resin and the polyolefin-based resin to satisfy properties of both of them, there is proposed a secondary-expandable expanded resin beads comprising a surface layer composed of an expanding polyolefin-based resin and a core part composed of an expanding polystyrene-based resin (Japanese Unexamined Patent Publication No. SHO 54(1979)-119563).
It is described that, according this expanded resin bead, since the blowing agent is retained in the expanding polystyrene-based resin in the core part, moldability is excellent. Further, it is described that an expanded molded article obtained from this expanded resin bead is excellent in stiffness, flexibility and low temperature properties.
However, the present inventors reproduced an experiment of the Publication and, when the blowing agent was impregnated into resin beads in which polystyrene-based resin beads were covered with a polyolefin-based resin, and this was pre-expanded, only the polystyrene-based resin in the interior was greatly expanded, while the polyolefin-based resin was slightly expanded, or was not expanded at all. As a result, a layer of the polyolefin-based resin and a layer of the polystyrene-based resin were peeled, and the expanded molded article of interest could not be obtained.
In addition to the aforementioned method, there is proposed a method of obtaining expanded resin beads in which the polystyrene-based resin was modified with the polyolefin-based resin, by impregnating polyethylene-based resin beads with a styrene-based monomer, and polymerizing this in an aqueous medium (e.g. Japanese Examined Patent Publication No. SHO59 (1984)-3487).
However, even by the aforementioned method, polystyrene-based resin pre-expanded beads which can provide an expanded molded article realizing simultaneously high chemical resistance, impact resistance, stiffness and expansion moldability could not be obtained.